1. Field of the Invention
This invention relates to a sealed storage battery having a safety valve for preventing excessive increase in the internal pressure of the battery, and more particularly to, a storage battery (secondary battery) of this kind having excellent resistance to liquid leakage, and a method for manufacturing the same.
2. Description of the Related Art
Sealed storage batteries are widely employed as power sources for small electric appliances because they are advantageous in that no supplement of electrolyte is required and no restriction is raised to their posture at the time of usage. The batteries of this type can roughly be classified into a cylindrical type and a square type in accordance with their outer shapes. The cylindrical type storage batteries, each having an electric power generator element obtained by spirally winding a positive sheet, a negative sheet and a separator interposed therebetween into a tubular form, can be manufactured with high efficiency. However, the cylindrical type batteries involve a problem that they tend to form a dead space when installed in appliances. The square type storage batteries eliminating such a problem include a generator element consisting of a plurality of positive plates and a plurality of negative plates, the positive and negative plates being laminated alternately via separators.
In these sealed storage batteries, to secure their hermetic sealing structure, the generator element is designed so as not to excessively increase the internal pressure of the battery by chemical reaction in the battery, and the open end of a container in which the generator element is received is sealed with a cover plate. However, it sometimes happens that the internal pressure increases excessively due to accidental short circuit in the battery or generation of gas upon overcharging the battery. In such cases, the container is likely to be damaged. To avoid this, a safety valve for preventing excessive increase in the internal pressure of the battery is disposed in a positive cap of the battery.
The sealed storage batteries of the aforementioned type are required to fix the positive cap to the cover plate in such a manner that electrical insulation and tightness (liquidtightness and gastightness) between the positive cap and the cover plate may be maintained, that the positive plates (positive sheets) are electrically connected to the positive cap, and that the safety valve disposed in the cap is communicated with the interior of the battery.
It is therefore conventionally known to seal the container using a sealing assembly which includes a hollow rivet. The hollow rivet is fitted via an insulating gasket in a center hole formed in the cover plate and is fixed to the cover plate, and the positive cap accommodating therein a safety valve is welded to the hollow rivet. Further, the positive cap is connected via the hollow rivet to the positive plate. With this sealing assembly, both fixing of the positive cap to the cover plate and electrical connection of the positive plate to the positive cap can be achieved by the hollow rivet, and also communication between the safety valve and the interior of the battery can be achieved by the bore of the hollow rivet.
FIGS. 1 and 2 show a square type sealed alkaline storage battery provided with the aforementioned type of sealing assembly. This storage battery is shaped into a rectangular parallelepiped form and has a reduced thickness, with a generator element 1a being housed in an outer can (container) 1. Further, an alkali electrolyte is charged into the container 1. The mark C denotes a sealing assembly to be mounted to the upper open end of the container 1. The sealing assembly C is welded along the entire circumference of the open end of the container 1 by means of laser welding.
More specifically, the sealing assembly C includes a cover plate 2 welded to the container 1. The cover plate 2 is formed at its center with a recess, and is formed with a through hole 2a extending through the center of the recess. A hollow rivet 3 is fitted in the through hole 2a via a gasket 6 for achieving electrical insulation and prevention of electrolyte leakage. The hollow rivet 3 has an annular head 3a and a cylindrical hollow shaft 3b formed integrally with the head 3a, an axial bore 3d being formed in the hollow shaft 3b. The head 3a is fitted in the recess of the cover plate 2 in such a way that the upper face of the head 3a is substantially flush with the upper surface of the cover plate 2. The axially inner end portion 3c of the hollow shaft 3b is caulked to an annular lead piece 7 disposed inside the cover plate 2. In achieving such caulking, an axial pressure is applied to the cover plate 2, gasket 6 and lead piece 7 by the head 3a and the inner end portion 3c of the hollow rivet 3. Thus, tightness between the opposing faces of the associated members among the cover plate 2, hollow rivet 3, gasket 6 and lead piece 7 can be enhanced, allowing the battery to have resistance to liquid leakage. Incidentally, a positive collector 1b connected to positive plates 1c is connected to the lead piece 7. In FIG. 2, reference numerals 1d, 1e and 1f denote a negative plate, a separator and a lead for the associated positive plate, respectively.
A positive cap 4 which includes a valve body 5 of the safety valve is fixed to the head 3a of the hollow rivet 3. The positive cap 4, which is, as a whole, formed into a box-like shape, has an open bottom. Notches 4b are formed at the respective lower ends of two side faces 4a of the cap 4. The valve 5 normally operates to block the outer opening of the bore 3d of the hollow rivet 3 to maintain gastightness of the battery. In the event of abnormal rise in the internal pressure of the battery for some reasons, the valve 5 opens to let the interior of the battery communicate to the outside of the battery via the bore 3d of the hollow rivet 3 and the notches 4b of the positive terminal 4. Thus, the gas in the storage battery can be exhausted through the bore 3d and notches 4b to the outside of the battery to lower the internal pressure of the battery.
A sealed alkaline storage battery has been proposed in Japanese Utility Model Publication No. 3-11803, which is provided with the aforementioned type of sealing assembly and which has further improved liquid leakage resistance. In this proposed battery (see FIG. 3), when the inner end portion 3c of the hollow rivet 3 is to be caulked in the process of manufacturing the battery, an axially intermediate portion of the hollow shaft 3b of the hollow rivet 3 is expanded radially outward so as to apply an axial pressure to the cover plate 2, gasket 6 and lead piece 7 by the head 3a and inner end portion 3c of the hollow rivet 3, and at the same time apply a radially outward pressure to the gasket 6 by the outer circumference of the intermediate portion of the hollow shaft 3b, thus improving tightness between the opposing surfaces of the associated members among the cover plate 2, hollow rivet 3 and gasket 6, in turn, resistance to liquid leakage in the battery.
By the way, to expand the hollow shaft 3b of the hollow rivet 3 radially outward as described above means to buckle the hollow shaft 3b. Accordingly, in the operation of caulking the hollow rivet in the proposed battery, a punch 8 is employed, which has a main body provided with a caulking end surface 8a, as indicated by the two-dotted chain line in FIG. 3, and which has a center shaft 8b formed integrally therewith and protruding axially outward from the caulking end surface 8a. Namely, an axial pressure sufficient to buckle the hollow shaft 3b is applied to the inner end portion 3c of the hollow rivet 3 by the caulking end surface 8a of the punch 8, with the center shaft 8b of the punch 8 being fitted in the bore 3d of the hollow rivet 3, to caulk the inner end portion 3c and at the same time expand the hollow shaft 3b radially outward.
However, it is extremely difficult to expand radially outward the hollow shaft 3b at a desired axial position utilizing the buckling phenomenon. Namely, even if the center shaft 8b of the punch 8 is fitted in the bore 3d of the hollow rivet 3 to properly position the punch 8 with respect to the hollow rivet 3 when the hollow rivet 3 is to be pressurized by the punch 8, the position to be buckled is likely to change depending on variable factors including a minor material variation in the hollow shaft 3b. Accordingly, the axial position of the hollow shaft 3b to be expanded, in turn, the position to which a radial force is to be applied by the expanded portion of the hollow shaft 3b to the gasket 6 is caused to change. Thus, it becomes difficult to maintain tightness between the opposing faces of the associated members among the cover plate 2, hollow rivet 3, gasket 6 and lead piece 7, which in turn makes it difficult to maintain the liquid leakage resistance of the battery at constant.
Meanwhile, if the hollow rivet 3 is pressurized in the axial direction to such a level as the hollow shaft 3b may be buckled, the lead piece 7 and cover plate 2 may sometimes be deformed, as shown in FIG. 3. If the lead piece 7 is deformed, a welding failure occurs when the lead piece 7 and the lead (shown by reference numeral if in FIG. 2) are welded in the subsequent step, or the lead piece 7 bites the gasket 6, as shown in the circle A of FIG. 3, so that the gasket 6 cannot achieve sufficient insulation between the lead piece 7 and the cover plate 2 to cause short circuit therebetween. Meanwhile, if the cover plate 2 is deformed as shown in the circle B of FIG. 3, there occurs a welding failure when the cover plate 2 is welded to the container 1 in the subsequent step, causing liquid leakage.